Resonance patterns

Electron delocalization

One molecule, several contributing structures

Resonance is a concept in chemistry used to describe molecules that cannot be represented by a single Lewis structure. Instead, they are depicted as a hybrid of two or more contributing structures, which differ only in the placement of electrons, not atoms. This phenomenon occurs when electrons, particularly π-electrons or lone pairs, are delocalized over adjacent atoms, creating stability through charge distribution. Electron delocalization reduces potential energy by spreading electron density across multiple bonds or atoms, resulting in equalized bond lengths and enhanced molecular stability.

In certain types of organic molecule, electrons are able to spread out over multiple atoms in order to stabilize the overall system. This requires conjugation, in which orbitals are aligned in patterns that allow for extended interaction — for example between non-bonding electrons and pi bonds, or pi bonds and empty p orbital(s). The examples below show a localized lone pair (left), where the non-bonding electrons are located only on the O atom, and a delocalized system (right) where the lone pair is shared between O and C. The presence of the pi system is essential for resonance delocalization, and spreading the electron density stabilizes both the charge and the molecule.

Source, sink, and Pattern A

Lone pair into an adjacent pi bond

There are really only three patterns involved with delocalizing a pair of electrons; here we will call them Pattern A, Pattern B, and Pattern C. To recognize resonance in a charged molecule it must contain an electron source and an electron sink. The former is either a non-bonded lone pair or a pi bond; the latter is usually an atom capable of accepting a lone pair, for example a second-row element or an empty p orbital. Typical examples are shown below for allyl-type anions, with the electron source being a lone pair and the sink being the terminal atom of the pi bond. The double-headed arrows may be used to describe the spreading of electron density in each case. This will be referred to as Pattern A.

Pattern B is the opposite: a carbocation next to a spare pair of electrons that help to stabilize the positive charge. The allylic carbocation is the simplest example, but it goes a long way in explaining a lot of second-semester chemistry. A heteroatom-stabilized cation, featuring O, N, S, etc. with a spare lone pair, will be essential in many mechanisms in Organic 2.

The allyl radical

One electron moves, and the other must be accounted for

The allyl radical is a related species that features a half-filled p orbital on carbon, such that C has seven electrons. Having a neighbouring pi bond will stabilize this species in a similar way to the allyl cation, except that only one electron is needed to help stabilize the radical; the second pi bond electron must therefore be accounted for in the second resonance structure by placing it on the third carbon of the allyl system. The typical allyl radical resonance structures are shown below (left), along with an orbital representation showing how the conjugated p orbitals interact.